test(e2e): add history panel playwright spec

Three scenarios: versions list appears after edits, diff shows changed
field, compare mode displays diff between two selected versions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/documents/[id]/+page.svelte

@@ -681,6 +681,7 @@ function versionLabel(v: VersionSummary, index: number): string {
 											<li>
 												<button
 													onclick={() => selectVersion(v.id)}
+													data-testid="history-version"
 													class="w-full py-2 text-left transition hover:bg-brand-sand/30 {selectedVersionId ===
 													v.id
 														? 'border-l-2 border-brand-mint pl-2'
@@ -717,12 +718,16 @@ function versionLabel(v: VersionSummary, index: number): string {
 									<p class="font-sans text-xs text-gray-400">{m.history_loading()}</p>
 								{:else if noDiff}
 									<div
+										data-testid="history-diff"
 										class="rounded-sm border border-brand-sand bg-white p-4 font-serif text-sm text-gray-400 italic"
 									>
 										{m.history_diff_no_changes()}
 									</div>
 								{:else if diffEntries.length > 0}
-									<div class="space-y-4 rounded-sm border border-brand-sand bg-white p-4">
+									<div
+										data-testid="history-diff"
+										class="space-y-4 rounded-sm border border-brand-sand bg-white p-4"
+									>
 										{#each diffEntries as entry (entry.field)}
 											<div>
 												<span